1. Field of the Invention
The present invention relates to a hydrogen generating system, which is equipped with a water electrolysis unit for performing electrolysis on water, such that when hydrogen generated by the water electrolysis unit is developed into high-pressure hydrogen at or above a predetermined pressure, the high-pressure hydrogen is extracted into a hydrogen supply passage via a back-pressure valve mechanism, and also relates to an operating method therefor.
2. Description of the Related Art
Recently, there has been proposed systems that supply electrical power or motive power using hydrogen as a fuel, for example, a fuel cell system or the like. For manufacturing hydrogen as a fuel, a water electrolysis apparatus is used, by which water is subjected to electrolysis in order to produce hydrogen (and oxygen).
In such a water electrolysis apparatus, generally, during electrolysis, oxygen is diffused through an electrolyte membrane due to concentration difference, and the diffused oxygen is mixed as impurity oxygen in the generated hydrogen on the order of several tens to several hundreds parts per million (ppm). Hence, there is a concern that such impurity oxygen will act as a catalytic poison in the fuel cell. Accordingly, concerning the degree of purity of hydrogen used in fuel cells, it is preferable for the impurity oxygen contained therein to be at or below 5 ppm, and more preferably, at or below 1 ppm.
Consequently, for removing impurity oxygen from within the generated hydrogen, for example, a method and apparatus for producing high-pressure hydrogen is known, as disclosed in Japanese Laid-Open Patent Publication No. 2007-100204. As shown in FIG. 5, such a high-pressure hydrogen producing apparatus comprises an oxygen high-pressure vessel 1, a differential pressure adjustment apparatus 2, a hydrogen high-pressure vessel 3, an electrolytic cell 4, a moisture-adsorption cylinder 5, a back-pressure valve 6, and a deoxidation cylinder 7.
Pure water inside the oxygen high-pressure vessel 1 is delivered to an anode side of the electrolytic cell 4 via a circulating pump 8, and by supplying electrical energy to the electrolytic cell 4 from a power source 9, electrolysis is carried out on the pure water. Oxygen, which is generated in the electrolytic cell 4 by electrolysis, is delivered to the oxygen high-pressure vessel 1, together with circulated and returned pure water from the circulating pump 8.
Hydrogen generated at the cathode of the electrolytic cell 4 is released to the interior of the hydrogen high-pressure vessel 3 together with permeated water. At this time, the pressure in the oxygen high-pressure vessel 1 and the pressure in the hydrogen high-pressure vessel 3 are kept equal by the differential pressure adjustment apparatus 2.
After hydrogen retained in the hydrogen high-pressure vessel 3 passes through the deoxidation cylinder 7 to remove oxygen contained in the hydrogen, hydrogen as a product gas is obtained as a result of removing moisture therefrom by the moisture-adsorption cylinder 5, which is connected to the back-pressure valve 6.
However, with the aforementioned Japanese Laid-Open Patent Publication No. 2007-100204, in particular, it is difficult for impurity oxygen contained within the hydrogen, which is generated upon start-up of the high-pressure hydrogen producing apparatus, to be eliminated reliably. Therefore, it has been necessary for a reaction system to be used for removing impurity oxygen generated upon start-up, which makes the structure of the apparatus complicated.
In addition, the pressure in the oxygen high-pressure vessel 1 and the pressure in the hydrogen high-pressure vessel 3 are kept at the same pressure. Accordingly, for example, even if the set pressure of the back-pressure valve 6 is changed, the impurity oxygen concentration within the hydrogen, which is generated in the hydrogen high-pressure vessel 3, cannot be reduced, and high-purity hydrogen cannot be produced efficiently.